As stated, on board upgrades are at the lowest bucket price so if the rooms are selling at a low bucket price you may as well buy from the ticket agent, but if the bucket is high try to buy on board. I appears to me that after the conductor takes your ticket you should go to the Lounge and wait for the Conductor to arrive to do his paperwork. I think such an "ambush" will work better than expecting him to get back to you.